The manufacture of many types of work pieces requires the substantial planarization of at least one surface of the work piece. Examples of such work pieces that require a planar surface include semiconductor wafers, optical blanks, memory disks, and the like. One commonly used technique for planarizing the surface of a work piece is the chemical mechanical planarization (CMP) process, a process commonly practiced in a multi-zonal processing apparatus. In the CMP process a work piece, held by a work piece carrier head, is pressed against a polishing pad and relative motion is initiated between the work piece and the polishing pad in the presence of a polishing slurry. The mechanical abrasion of the surface combined with the chemical interaction of the slurry with the material on the work piece surface ideally produces a surface of a desired shape, usually a planar surface. The terms “planarization” and “polishing,” or other forms of these words, although having different connotations, are often used interchangeably by those of skill in the art with the intended meaning conveyed by the context in which the term is used. For ease of description such common usage will be followed and the term “chemical mechanical planarization” will generally be used herein with that term and “CMP” conveying either “chemical mechanical planarization” or “chemical mechanical polishing.” The words “planarize” and “polish” will also be used interchangeably.
The construction of the carrier head of a CMP apparatus and the relative motion between the polishing pad and the carrier head as well as other process variables have been extensively engineered in an attempt to achieve a desired rate of removal of material across the surface of the work piece and hence to achieve the desired final surface shape. For example, the carrier head generally includes a flexible membrane that contacts the back or unpolished surface of the work piece and accommodates variations in that surface. A number of pressure chambers are provided behind the membrane so that different pressures can be applied to various zones on the back surface of the work piece to cause desired variations in polishing rate across the front surface of the work piece.
End point detection probes are often used to detect the completion of a polishing operation. The completion of the polishing operation is signaled or “called”, in accordance with a detection algorithm, as a function of the remaining material thickness. Upon detection of the end point signal, the CMP operation is either terminated immediately or after some prescribed delay denoted as an “over polish time.” The proper identification of endpoint is an important step in a CMP operation. Consider, for example, the removal of a copper layer from the surface of a semiconductor wafer as part of the process of forming a damascene pattern of interconnect metallization on that semiconductor wafer. If the endpoint is called too soon, an undesired layer of copper will remain on the semiconductor wafer causing an electrical short between unrelated metal conductors. If the endpoint is called too late, the polishing operation may cause damage to either the interconnect metal pattern or to the underlying insulator layers.
A number of different mechanisms and methods are available and commonly used for detecting the end point of a CMP operation. Such mechanisms and methods include optical end point detectors, eddy current monitors, measuring the drag experienced by the motors generating the relative motion between the work piece and the polishing pad, and the like. Each of these end point detection mechanisms and methods suffers from technical hurdles, especially when the layer being polished or removed is very thin. Specifically, calibration of the mechanism or method can be difficult at very thin layers, and noise sources can become a significant source of variation and error from work piece to work piece.
Accordingly, it is desirable to provide a chemical mechanical planarization method including an accurately determined end point. In addition, it is desirable to provide a method for accurate end point detection of a CMP operation. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.